1. Field of the Invention
The invention relates to telemetry of information through the earth's lithosphere. It is particularly adapted for telemetry of information from the bottom of an oil well to the surface of the earth during oil well drilling operations. The information telemetered may include but is not limited to the parameters of pressure, temperature, salinity, direction and deviation of the well bore, bit conditions, and logging data including resistivity of the various layers, sonic density, porosity, induction, self potential, and pressure gradients.
2. Description on the Prior Art
In previous oil well telemetry systems when it was desired to make measurements of important parameters at the bottom of the oil well, it was first necessary to pull up the drilling pipe section by section including the drilling bit to completely vacate the drilled hole. Sensors were then lowered down to the bottom of the well on a connected wire, the measurements were taken, the sensors and wire removed, and finally the bit and drilling pipe reassembled and put back into the hole. Obviously, such procedures were extremely expensive and time consuming since drilling operations had to be ceased each time measurements were to be made.
These problems have led to numerous attempts at oil well telemetry in which the drilling pipe and bit do not have to be removed from the well before measurements are made. Attempts have been made to telemeter data by means of sonic waves traveling through either the drilling pipe or through the drilling mud present both inside and surrounding the drilling pipe. Unfortunately, the drilling mud proved to be a strong sonic damper which destroyed the sonic waves before they could travel very far. Total depth attainable for telemetry with such systems was much smaller than minimally needed in a practical system.
Further attempts included installing a bifilar electric line either inside or outside of the drilling pipe or the casing pipe. Unfortunately, the mechanical stresses inside the well and the rocks and other debris brought up from the bottom of the well frequently destroyed the wire.
Another attempted system included a conductor inside of each section of drill pipe with transformer coupling between sections of pipe. Besides requiring expensive modifications to the drill pipe these systems proved unreliable in that magnetic coupling between sections was frequently hindered by mechanical misalignment between drill pipe sections and because of the attendant difficulty of aligning coupling coils with one another.
Still further attempts included one in which either the drilling pipe or casing pipe was used as one of the conductors in an electrical transmission system. In one such system, the earth itself formed the other conductor. Unfortunately, the conductivity of the earth is unpredictable and is frequently too low to make such a system practical at typical oil well depths. Still further such systems included a single wire along the casing pipe or drilling pipe. Such systems suffered from the problems discussed above with the bifilar type wire system. Both types of such systems suffered the additional common problem that the conductivity between pipe sections is greatly affected by the presence of contaminants on the pipe joints. Frequently the resistance of the pipe joints was too high to permit telemetry using any practical power levels.